Implement orderBy[rand] modifier.

src/Database/Esqueleto.hs

@@ -38,7 +38,7 @@ module Database.Esqueleto
     -- $gettingstarted
 
     -- * @esqueleto@'s Language
-    Esqueleto( where_, on, groupBy, orderBy, asc, desc, limit, offset, having
+    Esqueleto( where_, on, groupBy, orderBy, rand, asc, desc, limit, offset, having
              , sub_select, sub_selectDistinct, (^.), (?.)
              , val, isNothing, just, nothing, joinV, countRows, count, not_
              , (==.), (>=.), (>.), (<=.), (<.), (!=.), (&&.), (||.)

src/Database/Esqueleto/Internal/Language.hs

@@ -174,6 +174,9 @@ class (Functor query, Applicative query, Monad query) =>
   -- | @OFFSET@.  Usually used with 'limit'.
   offset :: Int64 -> query ()
 
+  -- | @ORDER BY random()@ clause.
+  rand :: expr OrderBy
+
   -- | @HAVING@.
   --
   -- /Since: 1.2.2/

src/Database/Esqueleto/Internal/Sql.hs

@@ -256,6 +256,7 @@ data SqlExpr a where
 
   -- A 'SqlExpr' accepted only by 'orderBy'.
   EOrderBy :: OrderByType -> SqlExpr (Value a) -> SqlExpr OrderBy
+  EOrderRandom :: SqlExpr OrderBy
 
   -- A 'SqlExpr' accepted only by 'set'.
   ESet :: (SqlExpr (Entity val) -> SqlExpr (Value ())) -> SqlExpr (Update val)
@@ -322,6 +323,8 @@ instance Esqueleto SqlQuery SqlExpr SqlBackend where
   asc  = EOrderBy ASC
   desc = EOrderBy DESC
 
+  rand = EOrderRandom
+
   limit  n = Q $ W.tell mempty { sdLimitClause = Limit (Just n) Nothing  }
   offset n = Q $ W.tell mempty { sdLimitClause = Limit Nothing  (Just n) }
 
@@ -903,7 +906,9 @@ makeOrderBy :: IdentInfo -> [OrderByClause] -> (TLB.Builder, [PersistValue])
 makeOrderBy _    [] = mempty
 makeOrderBy info os = first ("\nORDER BY " <>) $ uncommas' (map mk os)
   where
+    mk :: OrderByClause -> (TLB.Builder, [PersistValue])
     mk (EOrderBy t (ERaw p f)) = first ((<> orderByType t) . parensM p) (f info)
+    mk EOrderRandom = first ((<> "RANDOM()")) mempty
     orderByType ASC  = " ASC"
     orderByType DESC = " DESC"